Papain digestions of immunoglobulin splits the immunoglobulin into protein fragments identified as Fab (fraction antigen binding) and Fc (fraction crystallizing). The Fab fragment binds to antigen but does not yield agglutinating or precipitating aggregates. The Fab fraction has been separated and labeled with H.sup.3, I.sup.125, and peroxidase enzyme Immunochem 8(6) 509-73 (1971) and Journal of Histochemistry and Cytochemistry 22 (11) 1974, p. 999. These labeled Fab fragments have been used to label antigens on cells for ultrastructure autoradiography studies.